Retinoids are new drugs that can be used in the prevention of cancer of epithelial tissues. The proposed research involves investigation of the microsome-dependent biotransformation of all-trans-retinoic acid, 13-cis-retinoic acid, all-trans-N-ethylretinamide and all-trans-N-(4-hydroxyphenyl)retinamide. The objectives of these studies are: a) to assess the physiological significance of the interconversion of all-trans- and 13-cis-retinoic acids, b) to determine the biological activity of the biotransformation products derived from all-trans-N-ethylretinamide and all-trans-N-(4-hydroxyphenyl)retinamide using organ culture, c) to structurally identify the biotransformation products and d) to characterize the microsome-associated catalytic agent. Most of the work will be performed with unlabelled retinoids. High-pressure liquid chromatography (HPLC) will be the method used to determine the levels of retinoids and their biotransformation products in biological samples.